This invention relates to methods and solutions for preparing and applying metal phosphate coatings to a substrate.
Metal phosphate coatings are applied to a variety of substrates including metals and ceramics to perform a variety of useful functions, such as corrosion resistance against harsh chemical environments at elevated temperatures, as fiber coatings to provide toughening of high temperature composites, bio-active surfaces for implants, and, in general, surface modification of a substrate to obtain desirable electrical, chemical, thermal and mechanical properties. As one example, monazite or lanthanum phosphate coatings are applied to ceramic fibers and composites to provide structures for use at very high temperatures, as described in U.S. Pat. No. 5,665,463.
In general, it is known to apply a solution to a substrate containing the desired molar amounts of the metal and phosphate. The substrate is then heated to obtain a pure metal phosphate coating. In many cases, it is desired that the precursor solution have the capability of easily wetting and penetrating a substrate without separating into fractions. Another desirable property when dealing with high temperature metal phosphate coatings is the ability to convert the raw coating to the metal phosphate at relatively low temperatures, in order to minimize thermal degradation of the substrate.
Low temperature conversion to inorganic phosphate-based coatings is also desired on metal or alloy substrates for applications requiring corrosion resistance, thermal or electrical insulation, and other special properties. It is often difficult to synthesize suitable solutions with relatively inexpensive raw materials that yield the desired inorganic phase along with desirable film properties (uniform, dense, smooth and continuous). In many instances, the coatings are porous, multi-phase and require higher temperature heat treatment to obtain the desired single phase material. In applications requiring a mixed-metal phosphate-based coating (sodium zirconium phosphate, for example), the precursor solution should contain the respective metal and phosphate species to intimately mixed in order to avoid nucleation of a mixture of inorganic phases. Therefore, there is a need for developing appropriate precursor chemical solution that will yield metal or mixed-metal phosphate coatings with dense microstructure, morphology, and desired stoichiometry at relatively low temperatures.